As disclosed in Patent Document 1 (JP-A No. 5-304408), a conventional antenna device for portable terminals comprises a combination of an extensible and contractible external antenna which provides a high gain when extended and a greatly lowered gain when contracted, and a built-in antenna which provides a gain that is better than the external antenna as it is contracted, but lower than the external antenna as it is extended. The disclosed conventional antenna device provides good gains for transmitting and receiving radio waves, and can be used at two frequencies.
Many radio units and receivers that are commercially available at present which are compatible with frequencies in the VHF frequency band and frequencies in the UHF band employ a rod antenna, a helical antenna, or the like because wavelengths are quite long in the VHF band.
Radio units and receivers that are designed for better portability often employ earphones as an antenna.
JP-A No. 04-200047 (Patent Document 2) discloses an arrangement having two pairs of antennas disposed between two earphones and a cellular phone and selectively switchable by a switcher.
JP-A No. 61-281724 (Patent Document 3) discloses an arrangement which employs an earphone cord as a second antenna for diversity reception.
JP-A No. 2002-314450 (Patent Document 4) discloses an arrangement which employs earphone wires as two antennas and switches from one of the earphone wires to the other in the event of a reception failure during reception using one earphone wire.
Patent Document 1: JP-A No. 5-304408
Patent Document 2: JP-A No. 04-200047
Patent Document 3: JP-A No. 61-281724
Patent Document 4: JP-A No. 2002-314450